Since the early 1980's, silicon nitride based cutting tools have achieved an ever increasing role in the rough and finish machining of high volume cast iron parts. The growth of these cutting tools can be attributed to two factors. These factors being:                1. Silicon nitride cutting tool insert quality and performance improvements that provided improved reliability and operating parameter thresholds.        2. Machining company's acquisition of new high speed equipment, or rebuilt existing equipment that can utilize the high speed capability that silicon nitride cutting tools can provide.Prior to the introduction of 1980's silicon nitride cutting tool inserts, oxide based cutting tools were the ceramic cutting tool insert of choice within the cast iron machining industry. Oxide cutting tools of the pre-1980 era, however, were limited to the speed at which they could machine parts due to a propensity for thermal shock and the amount of impact they could endure due to their limited fracture toughness. Consequently, the overall acceptance of these oxide based tools was limited. The early 1980 era silicon nitride cutting tools provided the cast iron machining industry a more reliable ceramic cutting tool alternative. This was the start of the legitimization of ceramic inserts for the high volume machining, including milling operations, of cast iron products.        
The first generation of commercial silicon nitride cutting tool inserts was manufactured under U.S. Pat. No. 4,264,548 to Ezis, issued April 1981, and provided a process to manufacture a silicon nitride insert product that utilized yttria (Y2O3) and alumina (Al2O3) as sintering aids. The commercial cutting tools produced utilizing this art gave improved reliability and performance over the prior oxide based cutting tool insert products when used for production machining of cast iron products. U.S. Pat. No. 4,264,548 to Ezis, discusses using 4–12 w/o Y2O3 powder, and 0.50–2.5 w/o Al2O3 as sintering aids.
The second generation of commercial silicon nitride cutting tool inserts was manufactured under U.S. Pat. No. 4,652,276 to Burden, issued March 1987. This art teaches that using a combination of magnesia (MgO) and yttria (Y2O3), as sintering aids, further improves silicon nitride cutting tool insert impact resistance and insert reliability when machining cast iron products. U.S. Pat. No. 4,652,276 to Burden, discusses using 0.5 to 10.0 w/o MgO, and from 2.5 to 10 w/o Y2O3 as sintering aids.
The third generation of commercial silicon nitride cutting tool inserts is manufactured under U.S. Pat. No. 5,525,134 to Mehrotra et al., issued June 1996, which further refines the prior art of U.S. Pat. No. 4,652,276 to Burden by reducing the amount of magnesia (MgO), and yttria (Y2O3) sintering aids resulting in a further increase in cutting tool performance. Both U.S. Pat. No. 4,652,276 to Burden, and U.S. Pat. No. 5,525,134 to Mehrotra et al., teach that a ratio of or near 1 to 1, by weight, of yttria and magnesia as sintering aids in a silicon nitride ceramic can maximize cutting tool performance. Only the amounts of each of the sintering aids differ in the two inventions. U.S. Pat. No. 5,525,134 to Mehrotra et al., discusses using at least 0.2 w/o yttria and at least 0.2 w/o magnesia, wherein the sum of yttria and magnesia is less than 5 w/o as sintering aids.
There are many other commercial silicon nitride cutting tool insert compositions servicing the cast iron machining industry. Some are monolithic copies or offshoots of the arts previously mentioned. Some are sialon based products, while others are composite or whisker reinforced products. None of these commercial products, including those previously discussed, is compositionally similar or engineered as is the art disclosed within this invention.
Concurrent with the previously mentioned prior art cutting tool insert developments, other ceramic manufacturers were developing silicon nitride sintered products that include zirconia (ZrO2) along with magnesia (MgO), and yttria (Y2O3) as sintering aids. U.S. Pat. No. 4,560,669 to Matsuhiro et al., issued December 1985, and U.S. Pat. No. 5,120,328 to Pyzik, issued June 1992, disclose such art. However, there is no discussion of stoichiometric balance of the sintering aids with the system's silica. U.S. Pat. No. 4,560,669 to Matsuhiro et al., discusses using 2–15 w/o yttria, 0.5–15 w/o magnesia and 0.5–13 w/o zirconia as sintering aids. While U.S. Pat. No. 5,120,328 to Pyzik, discusses using 0.5–3.0 w/o magnesia, 1.0–6.0 w/o yttria and 0.2–3.0 w/o zirconia as sintering aids.
It is generally accepted that commercially available, ultra-fine, high purity alpha phase (90% plus alpha phase) silicon nitride powders, that are used for cutting tool insert manufacturing, contain small percentages of silicon dioxide (silica). This silica is thought to be in the form of a thin layer surrounding or coating the individual silicon nitride particles and is generally in the range of 2 w/o to 4 w/o of the gross silicon nitride powder's weight, depending upon the silicon nitride powder supplier and powder grade. The oxygen content of silicon nitride powder lots are analyzed for, and generally supplied, by the silicon nitride powder supplier. Since silica is 53.25 w/o oxygen, multiplying the analyzed oxygen weight percentage by 1.878, gives the weight percentage of silica in the particular silicon nitride powder lot. Knowing the exact percentage of silica in a silicon nitride powder is necessary to be able to stoichiometrically balance the sintering aids with the system's silica when forming the amorphous glass phase. This stoichiometric balance is the foundation of this invention.